Relaxation mechanism in NiFe thin films driven by spin angular momentum absorption throughout the antiferromagnetic phase transition in native surface oxides

We report an alternative mechanism for the physical origin of the temperature-dependent ferromagnetic relaxation observed in bare permalloy (NiFe) thin films. Through spin-pumping experiments, we demonstrate that the peak in the temperature dependence of NiFe damping can be understood in terms of enhanced absorption of spin angular momentum at the magnetic phase transition in native antiferromagnetic surface-oxidized layers. These results suggest some avenues for the investigation of an incompletely understood phenomenon in physics.

Figure 1

(a) Transmission electron microscopy image (TEM) and (b) energy-dispersive x-ray spectroscopy (EDX) data for a $\mathrm{Si}{\mathrm{O}}_2\text{/NiFe}$ sample. Samples were capped with Pt in preparation for the TEM experiment.

Figure 2

(a) Representative magnetization (M) vs field (H) hysteresis loops at different temperatures for a $\mathrm{Si}{\mathrm{O}}_2\text{/Cu/NiFe}$ sample. (b) Temperature $\left(T\right)$ dependence of the hysteresis loop shift $\left({\mathrm{H}}_{\mathrm{E}}\right)$.

Figure 3

(a) Diagrammatic representation of the spin-pumping experiment. (b) Temperature $\left(T\right)$ dependence of the NiFe layer Gilbert damping (α). The NiFe layer is surrounded by two, one, or no native oxide layers. When the NiFe is deposited directly on $\mathrm{Si}{\mathrm{O}}_2$ a 0.3-nm-thick NiFeOx naturally forms at the $\mathrm{Si}{\mathrm{O}}_2\text{/NiFe}$ interface, activated by the Ni and Fe atoms when interacting with the $\mathrm{Si}{\mathrm{O}}_2$ surface during sputter deposition. When the NiFe layer is left uncapped it naturally undergoes oxidation due to contact with air, resulting in a 1.6-nm-thick NiFeOx surface layer.

Figure 4

Temperature $\left(T\right)$ dependence of damping (α) of a NiFe ferromagnet directly coupled to a 1.6-nm-thick antiferromagnetic NiFeOx or separated by a 3-nm-thick Cu layer. In the $\mathrm{Si}{\mathrm{O}}_2\text{/Cu/NiFe/NiFeOx}$ sample, NiFeOx results from the native oxidation of NiFe(8 nm), creating a passivating 1.6-nm-thick layer. In the $\mathrm{Si}{\mathrm{O}}_2\text{/Cu/NiFe/Cu/NiFeOx}$ sample, NiFeOx results from complete native oxidation of a NiFe(1.6 nm) layer.

Figure 5

(a) Temperature $\left(T\right)$ dependence of the Gilbert damping (α) of the NiFe layer on temperature $\left(T\right)$ in $\mathrm{Si}{\mathrm{O}}_2\text{/NiFeOx}\left(t_{\mathrm{NiFeOx}}\right)\text{/NiFe/NiFeOx}\left(1.6\right)$ (nm) multilayers. (b) Thickness dependence of the critical temperature $\left(T_{\mathrm{crit}}\right)$ for the magnetic phase transition of the oxidized NiFe layer. Open circles represent data deduced from (a). Full squares represent data deduced from Fig. 3. Line fitting was based on the equation presented by Zhang et al. [44] in the thin-layer regime for a ${(\mathrm{NiO})}_{81}{\left(\mathrm{FeO}\right)}_{19}$ alloy.